1. Field of the Invention
This invention relates generally to an arousal level judging apparatus used for detecting an arousal level of an operator or a patient and more particularly, the invention relates to apparatus for detecting drowsiness during driving an automobile, which is applicable to a doze alarm system.
2. Description of the Prior Art
The main types of arousal level judging apparatus used for detecting the driver falling asleep which are known comprise; detection means having electrodes attached to a human body for detecting a skin potential level (hereinafter referred to as SPL) and a comparator for comparing the signal from the detection means with the predetermined value. Such technique has been described in Japanese patent application provisional publication No. 60-139539.
Generally, electrophysiological signals are known to have individual differences and variations within a day, i.e., diurnal variation. SPL also varies with diurnal variation, as shown in FIG. 2.
In the above-mentioned Prior art, in order to obtain a reference value to determine decrease ratio of SPL for judging a person falling asleep, a pre-experiment of sleep should be made before such detection of falling asleep is made actually because the reference value is required for compensation for the individual difference and diurnal variation.
However, an arousal level judging apparatus using the above-mentioned technique lacks ease in operation because such apparatus requires a user, for example, a car driver, to be subjected a pre-experiment of sleep. Although a compensation for general individual differences can be obtained through the above-mentioned pre-experiment, this compensation is ineffective for diurnal SPL variation because the reference to SPL will change with the passage of time.
Therefore, in the Prior art arousal level judging apparatus, there are drawbacks that it is difficult to obtain the reference to SPL, and that the reference should be determined at every predetermined intervals.